# VFD

#### mbohuntr

Joined Apr 6, 2009
430
I'm looking at a Seimens VFD driving a 60hz motor at 20Khz. Question... The end result is a duty cycle within the 60 hz signal. What is the range of frequencies, and how (if it is driven by Mosfet or IGBT) does this frequency translate into a duty cycle? It can't be a sine wave cause you will only get a 50% duty cycle...

#### mbohuntr

Joined Apr 6, 2009
430
Ok, after more research, I believe the 60hz is what is adjustable, 30 hz = half speed. Is the gate on the mosfet or IGBT fed a square pulse,varying frequency? How does the output become a simulated 30 hz sine wave?

#### jslimjeff

Joined Apr 16, 2009
1
A Variable Frequency drive in basics takes ac power and converts it to a DC voltage through the use of diode bridges. This allows the drive to either operate on single, two or three phases. The DC bus voltage is = 1.4 of the 3 phase power on the input. The DC bus uses capacitors and reactance to stabilize the voltage ripple. The DC bus then feeds two three IGBTs to convert the DC to a modified Sine wave. Pulse width modulation is the commonly used method today. Each of the gates are turned on with different duty cycles to simulate the voltage a the frequencey desired. As the IGBT is on longer the voltage will be higher and the opposite when it is left off.
Eaven though the voltage waveform will not look anything like a sine wave the current waveform on the device will resemble a sine wave.

Care must be taken by VFD manufactures on how fast the IGBT's are turned on and off. This causes DVTD issues and can lead to motor failure. Typical guidlines are that the Gates cannot turn on any faster than .1 us and the peak voltage of the overshoot cannot be greater than 1400 v.

This varies by different manufactures and can change based on reactance and capacitance in the drive. Motor lead lenght also plays a great factor in this as it changes impedance and capacitance of the circuit and can lead to voltage ringing.

Hope this helps.

#### mbohuntr

Joined Apr 6, 2009
430
So... current will relate to IGBT time on, and the control frequency is constant? The Diodes will provide the sinusiodal current wave... right?